Castable refractory materials are generally divided into two categories: conventional and low-cement. These castables are used in any applications requiring a durable lining capable of withstanding extremely high temperatures, for example, in the interior of furnaces and the ladles used in transferring molten iron during the steel manufacturing process. In general, a conventional castable material contains approximately 10% to 30% by weight of cement. A low-cement castable refractory material contains less than approximately 10% by weight of cement. Conventional castables may be applied in a number of ways: (1) by using forms to mold the conventional castable material and removing the forms after the material has set; (2) by a "dry method" of spray application, referred to in the trade as "gunnite"; and (3) by a "wet method" of spray application.
Of the three methods, forming and casting is the least desirable because of the greatly increased labor costs associated with building the forms. Additionally, some applications do not lend themselves to forming. For example, the ladles used in the steel mills to transport molten iron are roughly football-shaped. Building forms to accommodate the interior of the ladle would be impracticable.
The gunnite method of application consists of pneumatically conveying a dry refractory gunning mix into a compressed air stream and forcing it through a hose to a nozzle, adding water through a line at the nozzle and spraying the moistened mix onto the surface to be lined. Although gunnite eliminates the cost of forming and allows application to any surface regardless of shape or irregularities, it too suffers from a number of disadvantages. First,, the quality of the finished product is largely dependent on the skill of the nozzle operator. The operator must constantly control the flow of water to properly hydrate the dry mix. Sufficient water must be added to hydrate the cement in the mix, or the mixture will not properly adhere to the surface. The addition of too much water, however, weakens the compressive strength of the final product. Secondly, gunnite requires the use of a high volume of compressed air at the rate of approximately 250 to 700 cubic feet per minute. Because of the lack of thorough mixing, coarse aggregates contained in the gunning mix "rebound" from the surface. The amount of rebound material may be as high as one-third of the dry material passing through the gun. In addition to creating another factor that the nozzle operator must consider in controlling the amount of water added, the rebound of aggregates creates both a clean-up problem and an occupational safety issue. The high amount of dust generated during the gunnite process mandates that operators wear masks and respirators to avoid aspirating dust particles.
The wet method of spraying refractory material into place consists of pre-mixing dry refractory material with a measured amount of water, hydraulically conveying the pre-mixed material to a nozzle and spraying the material onto the surface to be lined. Since the material is pre-mixed with a measured amount of water, the nozzle operator is relieved of the task of controlling the amount of water added. The refractory material is typically mixed in a paddle mixer for several minutes, ensuring that the cement in the mix is thoroughly and uniformly wet. The wet method eliminates the problems of rebound and dust; the nozzle operator may accordingly work without a respirator. Additionally, the air volume required to place the material is approximately 80-90 cubic feet per minute, substantially less than that required gunnite process.
Low-cement castable refractory materials have properties far superior to conventional castables. Conventional castables form an end product having higher porosity than those formed with low-cement castables. In the steel mill applications, the higher porosity allows slag and molten iron a more effective surface to react with, thereby accelerating the degradation of the refractory. As the bond deteriorates, the refractory material loses its strength. The retained strength of low-cement refractory material is accordingly greater than that of conventional castables. Standard strength tests conducted by the applicant on a 60% alumina conventional castable and a 60% alumina low-cement castable yielded the following results:
______________________________________ Low-cement Low-cement Cold Modulus of Conventional Crushing Conventional Prefired Rupture, Modulus of Strength, Cold Crushing to .degree.F. psi Rupture, psi psi Strength, psi ______________________________________ 250 1260 1100 5360 4400 1500 2900 800 10900 4700 2000 2350 1100 9000 4000 2500 2800 1800 9700 6700 3000 3860 2000 12000 7200 ______________________________________
Although low-cement castables have physical properties that are superior to conventional castables, their use has been limited by a number of factors. First, because of the small amount of cement in the castable material, it must be pre-mixed with a measured amount of water for several minutes to activate the low-cement system. Additionally, the amount of water added to the low-cement castable material must be carefully controlled, as an excess of water will weaken the final refractory product. Since the "gunnite" method of application does not allow for mixing, and since the nozzle operator is required to determine the amount of water to add during the actual application, the dry method of spray application does not lend itself to use with a low-cement castable. Use of low-cement castables has been further limited by the fact that a low-cement castable is "thixotropic," i.e., it tends to flow when high shear energy is applied. Because of this property, low-cement castables previously could be placed only by forming and casting. Attempts to apply low-cement castables with a spray gun were unsuccessful, as the refractory material would flow off a vertical, semi-vertical or overhead surface. The inability to prevent the low-cement castable from running off of the surface resulted in limiting the use of low-cement castables to those applications in which forms could be used to cast the material. Applications in which forming was not feasible were limited to the gunning of conventional castables.